Seeing as this is different from the previous thread, I'll post a new one

The steampunk/fantasy-like setting has won over, and the basic setting for now centres around combined aerial/naval operations. The geography of the setting--lots of mountains with valleys around a large bay--is intended to reduce the need of large armies, as defensive positions in the form of fortresses at the end of valleys made it difficult for offensive operations.

Naval and riverine warfare was the norm until many of the different powers discovered a crystal that exerts a 'lifting' force twice as effective as a like quantity of helium deep underground in the mountains (the heat of the rock above, plus igneous intrusions, caused it). The raw form, which is a light blue sand, is similar but it would take 100lb to lift 101lb of mass--however, heat-treating it in furnaces produces the lifting crystal in sufficient quantities.

The reason I am trying to avoid large armies is because I am not sure how exactly to model infantry.

Marines seems to be a reasonable place to start, but they have a couple of problems--one, they have no effective attack value against anything unless they're on-board, and two--how would you buy them?

Fighters is another avenue, but then they wouldn't be able to perform infantry assaults on fortresses by breaching the walls and swarming throughout the fortress.

Breachers is another potential avenue to explore, but the nature of breachers are that they can fight on as fighters after unloading their marines. Maybe a new form of breacher could be devised that would be taken off the board when 'boarding' a land fortress or similar feature, but then can leave the fortress to carry on with the wider battle. I'm tempted to suggest something similar to fighters or slightly less in terms of the fighter modifier, but I'm not sure. It could also be used, potentially, to model mecha that can both board ships and fight as fighters.

Thoughts?